N,Q = map(int,input().split())
G = [[] for _ in range(N+1)]
Gnum = [0] * (N + 1)
edge = []
for i in range(Q):
    a,b = map(int,input().split())
    G[a].append((b,i))
    Gnum[b] += 1
    edge.append((a,b,i))

count = 0
top = []
stack = []
s = set()
for i in range(1,N+1):
    if Gnum[i] == 0:
        stack.append(i)
        count += 1
while stack:
    now = stack.pop()
    for v,i in G[now]:
        s.add(i)
        Gnum[v] -= 1
        if Gnum[v] == 0:
            count += 1
            stack.append(v)
import sys
if count == N:
    print(-1)
    exit()
for a,b,i in reversed(edge):
    if i in s:continue
    s.add(i)
    Gnum[b] -= 1
    if Gnum[b] == 0:
        count += 1
        stack.append(b)
        while stack:
            now = stack.pop()
            for v,j in G[now]:
                if j in s:continue
                s.add(j)
                Gnum[v] -= 1
                if Gnum[v] == 0:
                    count += 1
                    stack.append(v)
    if count == N:
        print(i+1)
        exit()